Many biological molecules and supramolecular systems, for example proteins, virus-like particles or attenuated viruses, are unstable and are susceptible to structural degradation and consequent loss of activity while stored, particularly in aqueous solutions. The processes involved in protein degradation can be divided into physical (i.e. processes affecting non-covalent interactions, such as loss of quaternary, tertiary or secondary structure, aggregation, surface adsorption) and chemical (i.e. processes involving a covalent change such as de-amidation, oxidation, disulphide scrambling etc.). The rates of the degradation processes are typically proportional to temperature. Biological molecules and supramolecular systems are therefore generally more stable at lower temperatures.
Metalloproteins are a class of proteins that contain one or more metal ions in their structure. The metal ion may be a part of a more complex chemical component (e.g. haem) which is bound within the protein structure. Alternatively, the metal ion may be bound directly to one or more amino acid side chains within the structure of the protein via various non-covalent interactions (co-ordinate interactions, hydrogen bonds, charge-charge interactions etc.). Whilst in some cases the metal may be essential for the protein's biological activity, in other cases it only plays a structural role. Whilst in some cases, for example in the Factor VIII molecule, the metal forms a bridge between two protein subunits, in other cases, for example in recombinant Anthrax protective antigen, the metal is confined within one subunit. The loss of the metal from the protein structure is likely to affect the function and/or the structure of the protein. Depending on the position of the metal within the protein molecule, the loss of the metal can lead to physical separation of key domains or to conformational change within one domain. Therefore, in order to maintain the native structure of the protein, it is very important to keep the protein in a formulation in which the binding interactions between the metal and the amino acid structure of the protein are maintained optimally.